xpath functions

I am getting error as “unknow method/function” when i try to access the xPath functions such as “Translate(), Starts-with()” either in javascript or vbscript.

Any one help me through this… how can i access all Xpath functions at client side/ server side functions ???

I just tried this
var xmlResult= Source.documentElement.selectSingleNode(“/employees/employee[starts-with(” + sSearchNode + “,” + sSearchText + “)]”);

getting error as “unknown method”